CN113980652A - Consistent-melting composite phase-change material and preparation method thereof - Google Patents

Abstract

The invention discloses a consistent-melting composite phase-change material and a preparation method thereof, belonging to the technical field of phase-change energy storage materials. The consistent melting composite phase change material comprises a phase change material and a heat conduction material, wherein the phase change material comprises a main phase change material and an auxiliary phase change material; wherein the main phase-change material is magnesium nitrate hexahydrate, and the content of the main phase-change material in the phase-change material is 60-70 mol%; the secondary phase-change material is lithium nitrate, and the content of the secondary phase-change material in the phase-change material is 30-40 mol%; and the heat conduction material accounts for 0-5 wt% of the sum of the main phase change material and the auxiliary phase change material, and the content is not 0. The phase change temperature of the main phase change material can be adjusted by utilizing the secondary phase, the problems of large supercooling degree and serious phase separation of a single hydrous salt phase change material can be solved by the eutectic phase change material, and a nucleating agent is not required to be added in the preparation process because the supercooling degree of the phase change material is extremely small; the phase-change material has good compatibility with aluminum and stainless steel 304, and is beneficial to wide application in the field of clean heating.

Description

A kind of consistent melting composite phase change material and preparation method

technical field

The invention belongs to the technical field of phase change energy storage materials, and relates to a uniform melting composite phase change material and a preparation method.

Background technique

At present, heating energy consumption accounts for more than 7% of my country's energy consumption. With my country's vigorous promotion of the energy revolution and the recently proposed carbon peaking and carbon neutrality goals, clean heating has rapidly risen as a major strategic demand in my country. According to the "Winter Clean Heating Plan for the Northern Region", by 2021, the clean heating rate in the northern region will reach 70%, replacing 150 million tons of scattered coal. In recent years, under my country's "coal-to-electricity" policy, most of the northern regions of China have implemented coal-to-electricity for clean heating in winter, aiming to reduce coal-fired heating. Due to the implementation of the "peak and valley electricity price" policy in most urban power grids in my country, the difference between day and night electricity prices is as much as 3-5 times. If the cheap electricity at night can be used, it can be converted into heat energy and stored and released during the daytime peak electricity consumption. For industrial production heat, residential heating, etc., it will greatly reduce the user's operating costs and reduce the use of traditional energy such as coal. It has a good role in the use of valley electricity phase change heat storage to realize the peak shifting and valley filling of the power grid, the stable and efficient operation of the power grid, and the improvement of the efficiency of the power grid. Phase-change heat storage utilizes materials to absorb (release) a large amount of latent heat during the phase change process to achieve the purpose of energy storage and controllable release. Compared with the water heat storage and solid heat storage methods currently used in clean heating projects, the phase change heat storage technology uses latent heat to store heat. Small and other advantages.

Inorganic phase change materials, especially hydrated salt phase change materials, have the advantages of high heat storage density, relatively high thermal conductivity, non-flammability, and low cost, so they have good application prospects in the field of clean heating. At present, crystalline hydrated salts are widely used in inorganic phase change materials. They all have relatively large phase transition heat and fixed melting point. When the temperature rises, the crystalline hydrated salt loses crystal water and causes the salt to dissolve and absorb heat. When the temperature is lowered, the reverse process occurs, absorbing crystallization and exothermic water. Compared with organic phase change materials, crystalline hydrated salts have the advantages of higher thermal conductivity, higher density and higher heat storage density per unit volume. However, crystalline hydrated salts have disadvantages such as high degree of supercooling, phase separation and easy agglomeration.

The inorganic hydrated salts that can be used for building heating mainly include aluminum ammonium sulfate dodecahydrate, barium hydroxide octahydrate, and sodium acetate trihydrate. Among them: the phase transition temperature of aluminum ammonium sulfate dodecahydrate is 95 ℃, but the phase transition temperature is high for specific applications, and it has the problems of weak acidity and high degree of supercooling; the phase transition temperature of barium hydroxide octahydrate (78 ℃) It has a high latent heat of phase transition (265.7kJ/kg), but it is corrosive to a certain extent and belongs to hazardous chemicals; the phase transition temperature of sodium acetate trihydrate is 58.0 °C, the latent heat value is high, and its heat storage temperature area is relatively low, usually Suitable for radiant heating. Therefore, the existing types of hydrated salt phase change materials suitable for the heating needs of radiators can be relatively low in selectivity.

At present, the heating and cooling terminals of buildings in my country are mainly radiator heating and radiant heating. Taking the system with hot water as the heat medium as an example, according to GB50736-2012 Design Specification for Heating, Ventilation and Air Conditioning of Civil Buildings, the temperature of water supply for floor radiant heating should be 35-45°C, and should not exceed 60°C; the temperature of water supply for radiator heating should be 75°C , should not exceed 80 ℃. The phase transition temperature and heat release rate of the material in the heat storage system should match the working temperature of the heat dissipation end, otherwise the efficiency of the heating system will be reduced.

The phase transition temperature of magnesium nitrate hexahydrate considered in this application is 89°C, the latent heat of phase transition is as high as 150kJ/kg, and the cost is low and environmentally friendly. Although it can meet the fields of radiator heating and solar hot water, there are still heat storage The technical problem that the working temperature of the end does not match the working temperature area of the heat dissipation end.

In particular, when magnesium nitrate hexahydrate is used as a single-phase hydrated salt phase change material at the end of the radiator, the inventor found that there is a certain difference in the temperature at the end of the radiator between the phase change temperature zone and the heating ventilation and air conditioning design of civil buildings. On this basis, how to control the phase transition temperature zone of magnesium nitrate hexahydrate, so that the working temperature of the heat storage end is highly matched with the working temperature zone of the heat dissipation end, and how to meet the specification design requirements for specific applications of clean heating is an urgent problem to be solved.

SUMMARY OF THE INVENTION

The technical problem solved by the present invention is how to solve the technical mismatch between the phase-change temperature region of magnesium nitrate hexahydrate as a single-phase hydrated salt phase-change material and the temperature at the end of the radiator.

In order to solve the above-mentioned technical problems, the present invention provides the following technical solutions:

A consistent melting composite phase change material, the consistent melting composite phase change material includes a phase change material and a thermally conductive material, and the phase change material includes a primary phase change material and a secondary phase change material; wherein,

The main phase change material is magnesium nitrate hexahydrate, and the content in the phase change material is 60-70mol%;

The secondary phase change material is lithium nitrate, and the content in the phase change material is 30-40mol%;

The thermally conductive material has a content of 0-5 wt % of the sum of the main phase change material and the auxiliary phase change material, and the content is not 0.

Preferably, the thermal conductivity of the thermally conductive material is 50-200 W·m ⁻¹ ·K ⁻¹ .

Preferably, the thermally conductive material is one or more of a metal matrix, a metal oxide matrix and a carbon material.

Preferably, the metal matrix is selected from one or more of foamed silver, foamed copper, nano-silver and nano-copper; the metal oxide matrix is selected from one or more of nano-alumina and nano-titanium oxide ; The carbon material is selected from one or more of carbon nanotubes, graphene, nanographite, expanded graphite or cellulose nanofibers.

Preferably, the preparation method of the consistent melting composite phase change material comprises the following steps:

S1, according to the composition and content selection of described consistent melting phase change material, take by weighing magnesium nitrate hexahydrate and lithium nitrate;

S2, pour the magnesium nitrate hexahydrate and lithium nitrate weighed in step S1 into the container and stir evenly, put into a magnetic stirrer and seal the container;

S3, heating the container in step S2 in a water bath on a magnetic heating stirrer, setting the temperature to 90-100° C., after the sample is melted, turn on the magnetic stirring and fully stir for 0.5-1 h to obtain a eutectic material;

S4, cooling and solidifying the eutectic material obtained in step S3, thereby obtaining a uniform melting composite phase change material with a phase transition temperature of 75±5°C.

Preferably, the preparation method of the consistent melting composite phase change material further comprises the following steps:

S5, heating the uniform melting composite phase change material at 75±5°C obtained in step S4 in a beaker at 75-100°C until it is completely melted;

S6, slowly pour the thermally conductive material containing 0-5wt% of the sum of the main phase change material and the secondary phase change material into the beaker of S5, heat and stir for 0.5-1h, until completely mixed to obtain a mixed molten material;

S7, when the thermally conductive material in step S6 is granular, take out the mixed molten material in step S6, and perform ultrasonic vibration for 0.5-1 h; when the thermally conductive material in step S6 is porous, then ultrasonic vibration treatment is not required;

S8. The material obtained in step S7 is cooled and solidified to room temperature, thereby obtaining a uniform melting composite phase change material with higher thermal conductivity and a required phase transition temperature of 75±5°C.

Preferably, in the step S2, when the preparation amount of magnesium nitrate hexahydrate and lithium nitrate is more than 50kg, the stirring method adopts mechanical stirring; when the preparation amount of magnesium nitrate hexahydrate and lithium nitrate is less than 0.5kg, the stirring method adopts magnetic stirring. ; When the preparation amount of magnesium nitrate hexahydrate and lithium nitrate is between 0.5-50kg, the stirring method adopts mechanical stirring or magnetic stirring.

Preferably, in the step S6, when the preparation amount of magnesium nitrate hexahydrate and lithium nitrate is more than 50kg, the stirring method adopts mechanical stirring; when the preparation amount of magnesium nitrate hexahydrate and lithium nitrate is less than 0.5kg, the stirring method adopts magnetic stirring. ; When the preparation amount of magnesium nitrate hexahydrate and lithium nitrate is between 0.5-50kg, the stirring method adopts mechanical stirring or magnetic stirring.

Preferably, the phase transition temperature of the magnesium nitrate hexahydrate is 89° C., and the latent heat of phase transition is as high as 150 kJ/kg.

Preferably, the phase transition temperature of the consistent melting composite phase change energy storage material is 75±5°C, the latent heat of phase transition is not less than 170kJ/kg, and the phase transition process is reversible.

The above-mentioned technical solutions provided by the embodiments of the present invention have at least the following beneficial effects:

In the above scheme, in order to solve the contradiction between the phase transition temperature of a single inorganic hydrated salt and a specific application temperature region, a consistent melting composite phase change material is prepared by adding one or more eutectic materials synthesized by phase change materials, which can effectively change its phase transition temperature. However, the research on the composite phase change material suitable for the terminal heat sink (70-80 °C) is rarely reported.

The consistent melting composite phase change material includes a phase change material and a thermally conductive material, and the phase change material includes a main phase change material and a secondary phase change material, wherein the main phase change material is magnesium nitrate hexahydrate, the secondary phase change material is lithium nitrate, and the thermally conductive material It is a material with a thermal conductivity of 50-200W·m ^-1 ·K ^-1 .

The uniform melting composite phase change material does not need to add a nucleating agent because of its extremely small degree of undercooling, which is about 1.76 °C.

The phase change temperature zone of the uniform melting composite phase change material is 75±5℃, which meets the design requirements of radiator heating in the building heating design specification, and the phase change material has good compatibility with aluminum and stainless steel 304, and can be used in large-scale hydration The salt phase change heat storage and clean heating field is in line with the country's current strategic plan for carbon peaking and carbon neutrality.

Description of drawings

In order to illustrate the technical solutions in the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without creative effort.

Fig. 1 is the DSC chart of the consistent melting composite phase change material of Example 2 of the present invention;

Fig. 2 is the heating-cooling curve diagram of the uniform melting composite phase change material of Example 2 of the present invention;

FIG. 3 is a comparison diagram of the corrosion situation of carbon steel, aluminum, 304 stainless steel and brass four metal surfaces by the consistent melting composite phase change material of Example 2 of the present invention; wherein a, b, c, d are respectively the carbon before corrosion Steel, aluminum, 304 stainless steel and brass four metal surfaces, e, f, g, h are carbon steel, aluminum, 304 stainless steel and brass four metal surfaces after corrosion for 360h respectively.

Detailed ways

In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, the following will be described in detail with reference to the accompanying drawings and specific embodiments.

Example 1

Weigh a total of 50g of materials, wherein magnesium nitrate hexahydrate accounts for 60mol%, lithium nitrate accounts for 40mol%, mix well, put in a magnetic stirrer and seal the container; heat the water bath to a constant 95 ℃ on the constant temperature magnetic stirrer, wait for it After being completely melted into a liquid state, magnetically stirred for 30 minutes to ensure uniform mixing until the mixture was in a uniform liquid state, and there were obvious liquid flow traces in the shaking beaker; cooled and solidified to room temperature to obtain a consistent molten phase change material of magnesium nitrate hexahydrate-lithium nitrate.

Example 2

Weigh 50g of the material in Example 1, heat it to a liquid state in a beaker, add 2.5wt% expanded graphite to the beaker, heat and stir for 30min until fully mixed to obtain a mixed molten material, and then cool and solidify to room temperature , to obtain magnesium nitrate hexahydrate-lithium nitrate-expanded graphite composite phase change material.

Example 3

Weigh a total of 60g of materials, wherein magnesium nitrate hexahydrate accounts for 70mol%, lithium nitrate accounts for 30mol%, mix well, put in a magnetic stirrer and seal the container; heat the water bath to a constant 93 ℃ on the constant temperature magnetic stirrer, wait for it After being completely melted into a liquid state, magnetically stirred for 40 minutes to ensure uniform mixing, until the mixture was in a uniform liquid state, and there were obvious liquid flow traces in the shaking beaker; cooled and solidified to room temperature to obtain a consistent molten phase change material of magnesium nitrate hexahydrate-lithium nitrate.

Example 4

Weigh 60 g of the material in Example 1, heat it to a liquid state in a beaker, add 1.5 wt% silver foam to the beaker, heat and stir for 40 min until fully mixed to obtain a mixed molten material, and then cool and solidify to room temperature , to obtain magnesium nitrate hexahydrate-lithium nitrate-silver foam composite phase change material.

Example 5

Weigh a total of 20kg of materials, wherein magnesium nitrate hexahydrate accounts for 65mol%, and lithium nitrate accounts for 35mol%, mix well, put in a magnetic stirrer and seal the container; on the constant temperature magnetic stirrer, the water bath is heated to a constant 97 ℃, and the After being completely melted into a liquid state, magnetic stirring was performed for 55 minutes to ensure uniform mixing until the mixture was in a uniform liquid state, and there were obvious liquid flow traces in the shaking beaker; cooled and solidified to room temperature to obtain a consistent molten phase change material of magnesium nitrate hexahydrate-lithium nitrate.

Example 6

Weigh 20kg of the material in Example 1, add 4.5wt% of nano-alumina after heating it to a liquid state, heat and stir for 55min until fully mixed to obtain a mixed molten material, carry out ultrasonic vibration for 40min, then cool and solidify to room temperature, The magnesium nitrate hexahydrate-lithium nitrate-nano alumina composite phase change material is obtained.

Example 7

Weigh a total of 40kg of materials, wherein magnesium nitrate hexahydrate accounts for 63mol%, lithium nitrate accounts for 37mol%, mix well, heat to a constant 100 ℃ in a water bath on a mechanical stirrer, and mechanically stir for 45min after it is completely melted into a liquid state to ensure uniform mixing , until the mixture is in a uniform liquid state, and the shaking beaker has obvious liquid flow traces; it is cooled and solidified to room temperature to obtain a consistent molten phase change material of magnesium nitrate hexahydrate-lithium nitrate.

Example 8

Weigh 40kg of the material in Example 1, add 3.5wt% of nano-silver after heating it to a liquid state, heat and mechanically stir for 45min until fully mixed to obtain a mixed molten material, carry out ultrasonic vibration for 50min, then cool and solidify to room temperature, The magnesium nitrate hexahydrate-lithium nitrate-nano-silver composite phase change material is obtained.

Example 9

Weigh a total of 60kg of materials, wherein magnesium nitrate hexahydrate accounts for 67mol%, lithium nitrate accounts for 33mol%, mix well, heat to a constant 98 ℃ in a water bath on a mechanical stirrer, and mechanically stir for 55min after it is completely melted into a liquid state to ensure uniform mixing. , until the mixture is in a uniform liquid state, and the shaking beaker has obvious liquid flow traces; it is cooled and solidified to room temperature to obtain a consistent molten phase change material of magnesium nitrate hexahydrate-lithium nitrate.

Example 10

Weigh 60kg of the material in Example 1, add 3.3wt% cellulose nanofibers after being heated to a liquid state, heat and mechanically stir for 55min until fully mixed to obtain a mixed molten material, carry out ultrasonic vibration for 60min, then cool and solidify to At room temperature, magnesium nitrate hexahydrate-lithium nitrate-cellulose nanofiber composite phase change material was obtained.

Performance Testing

The phase change materials obtained in the above-mentioned Examples 1 to 10 are tested by the following test methods, and the test results are shown in Table 1:

S1. Using a differential scanning calorimeter (DSC), under a nitrogen flow of 50ml/min and a heating and cooling rate of 10°C/min, measure the phase transition temperature and phase transition latent heat of the phase change materials obtained in Examples 1 to 10 ; The phase transition temperature and the latent heat of phase change of the phase change material in Example 2 are shown in Figure 1; in Figure 1: the temperature corresponding to the inflection point of the curve is the phase transition temperature of the phase change material obtained in Example 2, and the horizontal line and the curve are surrounded by The peak area of is the latent heat of phase change of the phase change material obtained in Example 2; it can be concluded that the phase change temperature of the phase change material obtained in Example 2 is 72.89 ° C, and the latent heat of phase change is 171.79 J/g.

S2. Heating and cooling curve: Pour it into a test tube in a liquid state, insert a thermocouple, and after it cools to room temperature, place it in a 95°C water bath to heat it again. After the temperature reaches the water bath temperature, place it in the air to After cooling, the heating and cooling curves of the phase change materials obtained in Examples 1 to 10 can be obtained; wherein the heating and cooling curves of the phase change materials in Example 2 are shown in Figure 2; from this, it can be concluded that the phase change materials obtained in Example 2 have obvious The exothermic platform, suitable phase transition temperature and extremely small subcooling degree can be widely used in the field of phase change heat storage and clean heating.

S3. Corrosion test: prepare carbon steel, aluminum, 304 stainless steel and brass four kinds of metal corrosion sheets, put them in the test tubes containing the phase change materials obtained in Examples 1 to 10, and heat them in an oven at 85 ° C for 360h , after 360h, take it out to observe the metal surface condition to judge its corrosiveness; the metal surface condition before and after the corrosion of carbon steel, aluminum, 304 stainless steel and brass by the phase change material of Example 2 is shown in Figure 3; From this, it can be concluded that the phase change material obtained in Example 2 is less corrosive to aluminum and stainless steel 304, especially to stainless steel 304.

S4, thermal conductivity: use the thermal constant analyzer TPS 2500S of Sweden Hot Disk company to test the thermal conductivity of the phase change materials of Examples 1-10;

The process parameters of the phase change material measured in specific examples 1-10 are shown in Table 1 below.

Table 1 Thermal conductivity of Examples 1-10

In the above scheme, the consistent melting composite phase change material includes a phase change material and a thermally conductive material, and the phase change material includes a main phase change material and a secondary phase change material, wherein the main phase change material is magnesium nitrate hexahydrate, and the secondary phase change material is nitric acid. Lithium, the thermally conductive material is a material with a thermal conductivity of 50-200W·m ^-1 ·K ^-1 .

The uniform melting composite phase change material does not need to add a nucleating agent because of its extremely small degree of undercooling, which is about 1.76 °C.

The phase change temperature zone of the uniform melting composite phase change material is 75±5℃, which meets the design requirements of radiator heating in the building heating design specification, and the phase change material has good compatibility with aluminum and stainless steel 304, and can be used in large-scale hydration The salt phase change heat storage and clean heating field is in line with the country's current strategic plan for carbon peaking and carbon neutrality.

The above are the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can be made. These improvements and modifications It should also be regarded as the protection scope of the present invention.

Claims (10)

1. The consistent melting composite phase change material is characterized by comprising a phase change material and a heat conduction material, wherein the phase change material comprises a main phase change material and an auxiliary phase change material; wherein,

the main phase-change material is magnesium nitrate hexahydrate, and the content of the main phase-change material in the phase-change material is 60-70 mol%;

the secondary phase-change material is lithium nitrate, and the content of the secondary phase-change material in the phase-change material is 30-40 mol%;

and the heat conduction material accounts for 0-5 wt% of the sum of the main phase change material and the auxiliary phase change material, and the content is not 0.

2. The coherent melt composite phase change material of claim 1, wherein the thermally conductive material has a thermal conductivity of 50-200W-m^-1·K^-1。

3. The coherent melt composite phase change material of claim 2, wherein the thermally conductive material is one or more of a metal matrix, a metal oxide matrix, and a carbon material.

4. The coherent melt composite phase change material of claim 3, wherein the metal matrix is selected from one or more of silver foam, copper foam, nano silver, and nano copper; the metal oxide matrix is selected from one or more of nano aluminum oxide and nano titanium oxide; the carbon material is selected from one or more of carbon nano tube, graphene, nano graphite, expanded graphite or cellulose nano fiber.

5. A method for preparing an coherent melt composite phase change material according to any of claims 1 to 4, wherein the method for preparing the coherent melt composite phase change material comprises the steps of:

s1, weighing magnesium nitrate hexahydrate and lithium nitrate according to the components and content selection of the consistent melting phase-change material;

s2, pouring the magnesium nitrate hexahydrate and the lithium nitrate weighed in the step S1 into a container, uniformly stirring, putting a magnetic stirrer, and sealing the container;

s3, heating the container in the step S2 on a magnetic heating stirrer in a water bath at the set temperature of 90-100 ℃, and after the sample is melted, starting magnetic stirring and fully stirring for 0.5-1h to obtain a eutectic material;

and S4, cooling and solidifying the eutectic material obtained in the step S3 to obtain the consistent melting composite phase change material with the phase change temperature of 75 +/-5 ℃.

6. The method of claim 5, further comprising the steps of:

s5, heating the consistent melting composite phase change material with the temperature of 75 +/-5 ℃ prepared in the step S4 in a beaker with the temperature of 75-100 ℃ until the consistent melting composite phase change material is completely melted;

s6, slowly pouring a heat conduction material with the content of 0-5 wt% of the sum of the main phase change material and the auxiliary phase change material into the beaker of S5, heating and stirring for 0.5-1h until completely mixing to obtain a mixed molten material;

s7, when the heat conducting materials in the step S6 are granular, taking out the mixed molten materials in the step S6, and carrying out ultrasonic oscillation for 0.5-1 h; when the heat conducting material in the step S6 is porous, ultrasonic oscillation processing is not required;

s8, cooling and solidifying the material obtained in the step S7 to room temperature, and thus obtaining the consistent melting composite phase change material with higher heat conductivity and required phase change temperature of 75 +/-5 ℃.

7. The method for preparing the phase change material according to claim 5, wherein in step S2, when the preparation amounts of magnesium nitrate hexahydrate and lithium nitrate are more than 50kg, the stirring manner is mechanical stirring; when the preparation amounts of the magnesium nitrate hexahydrate and the lithium nitrate are below 0.5kg, magnetic stirring is adopted in a stirring mode; when the preparation amounts of the magnesium nitrate hexahydrate and the lithium nitrate are between 0.5 and 50kg, the stirring mode adopts mechanical stirring or magnetic stirring.

8. The method for preparing the phase change material according to claim 6, wherein in step S6, when the preparation amounts of magnesium nitrate hexahydrate and lithium nitrate are more than 50kg, the stirring manner is mechanical stirring; when the preparation amounts of the magnesium nitrate hexahydrate and the lithium nitrate are below 0.5kg, magnetic stirring is adopted in a stirring mode; when the preparation amounts of the magnesium nitrate hexahydrate and the lithium nitrate are between 0.5 and 50kg, the stirring mode adopts mechanical stirring or magnetic stirring.

9. The method of claim 5, wherein the magnesium nitrate hexahydrate has a phase transition temperature of 89 ℃ and a latent heat of phase transition of up to 150 kJ/kg.

10. The method for preparing the uniform melting composite phase change material as claimed in claim 5, wherein the phase change temperature of the uniform melting composite phase change energy storage material is 75 ± 5 ℃, the latent heat of phase change is not less than 170kJ/kg, and the phase change process is reversible.

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